A nuclear reactor of the boiling water type employs a cylindrical shell in the lower part of which the reactor core is located. A continuous total circulation of cooling water internally of the reactor serving to remove heat from the reactor core is composed of the delivery of an external circulating pump to an array of jet pumps inside the shell, which jet pumps are energized by the flow delivered by the circulating pump to the jet pumps inside the shell. The jet pumps energized by the flow of water from the main circulating pumps induce a circulation through the core much greater in volume per unit of time than the flow delivered by the external circulating pump to the jet pumps.
The internal jet pumps, which are supplied internally of the shell with recirculating water by the external pumps, create internal circulation in the shell containing the reactor core. In other words, the external pumps activate the jet pumps whereby an internal or local circulation of much greater volume than that which passes through the external pumps flows in thermal contact with the reactor core in said shell.
The problem presented is that stopping the external circulation and opening the external circulation path of the recirculating pumps, which sometime become necessary, allows backward flow of the water contained in the shell out through the inlet of the contained jet pumps to the outside of the casing endangering the reactor. The tool of the present invention makes possible the closing off of the jet nozzles individually and prevents outflow of the contents of the shell, thereby avoiding the necessity for taking the reactor out of service and removing the fuel bundles.